NOTES : It VS THAT

If "it" and "that" are used as a pronoun, how do I choose between them? For example,

The dictionary is very helpful; without it, I don't know what the word means at all.
The dictionary is very helpful; without that, I don't know what the word means at all.

On the gmat, and in formal written english in general, you can't use the so-called 'pointing words.' those are the four words 'this', 'that', 'these', and 'those', used by themselves as pronouns. big fat no-no. the second one of these two sentences is, therefore, incorrect. -- the above is of course not meant to say that those four words are forbidden in general. you can, of course, use 'this', 'that', 'these', and 'those' as adjectives (these people, that book, etc.), or in specialized constructions wherein parallelism lends them meaning (such as 'beethoven's symphonies were more revolutionary than were those of bach').

Examples:
example-quetzalcoatlus
example-living-in-freestanding-house